The overall objective of the proposed work is to gain additional information on the manner in which light-dark signals interact with circadian oscillators to control the timing of ovulation. In one series of experiments, attempts will be made to develop a lighting regimen that will constantly cause the active portion of the locomotor activity rhythm of rats to split into two circadian components, i.e. an evening (E) component with free-running period longer than 24 h and a morning (M) component with a free-running period shorter than 24 h. This will make it possible to determine whether the control of estrous cyclicity and the timing of ovulation is related primarily to the E, the M, or both components. In a second series of experiments we will determine the responsiveness of the locomotor activity rhythm and the ovulation rhythm to light cues delivered at specific intervals throughout the 24 h day. With this information phase-response curves for the locomotor and ovulation rhythm of the rat can be constructed. These will provide basic characteristics of circadian oscillators which control hormonal and behavioral rhythms in the rat and possibly other species.